The composition of ovarian cyst fluid has received relatively little attention. It is known that cyst fluid contains a number of proteins and glycoproteins. We have used contemporary biochemical techniques to evaluate protein secretion of ovarian epithelium. The protein composition of 14 ovarian cyst fluids from either cystadenomas or cystadenocarcinomas, and conditioned media from 7 ovarian carcinoma cell lines were analyzed by SDS- PAGE and Western immunoblots. The major protein common to all of the above samples was a 65 KDa protein that constituted 40-77% of ovarian cyst fluid and 19-38% of conditioned media. By Western blot analysis this protein was immunologically related to albumin. Moreover immunoreactivity to anti- albumin antibody was demonstrated in ovarian epithelium in vivo. Thus the major secretory product of ovarian epithelium is an albumin-like protein. It is noteworthy that the ovarian carcinoma cell lines secrete the same or similar protein as that found in ovarian cyst fluids. This may suggest an osmotic mechanism for cyst enlargement in ovarian cystadenomas. Further, these results caution against considering albumin production to be exclusively a function of hepatocytes.